Macromolecules, Vol.41, No.6, 2228-2234, 2008
Synthesis, folding, and association of long multiblock (PEO23-b-PNIPAM(124))(750) chains in aqueous solutions
A thermally sensitive ultralong multiblock copolymer, [poly(ethylene oxide)(23)-b-poly(N-isopropylacrylamide)(124)](750) (M-w = 1.78 x 10(7) g/mol and M-z/M-w = 1.49), was prepared using the oxidative coupling of two mercapto groups at the two ends of triblock PNIPAM(62)-b-PEO23-b-PNIPAM(62) (M-n,M-PEO = 1.0 x 10(3) g/mol and M-n,M-PNIPAM = 1.4 x 10(4) g/mol) copolymer chains. The folding of individual multiblock copolymer chains in an extremely dilute solution (10(-6) g/mL) was studied by laser light scattering (LLS). Moreover, the association of multiblock and triblock copolymer chains in relatively concentrated aqueous solutions (10(-3) g/mL) was also comparatively studied by a combination of LLS, fluorescence spectrometry, and microcalorimetry. We found that in the single-chain folding process the average radius of gyration (< R-g >) remains a constant in one heating-and-cooling cycle, but the average hydrodynamic radius (< R-h >) decreases as the solution temperature increases. Our result reveals that the single-chain folding undergoes two stages at similar to 32 and similar to 40 degrees C, presumably due to the successive contraction of thermally sensitive PNIPAM segments in the middle-around each hydrophobic S-S coupling point and near the hydrophilic PEO block. Each PNIPAM block collapses into a small globule (bead) stabilized by the two attached PEO blocks on the chain backbone, a string-bead conformation, which makes the chain thicker and more extended. The association of multiblock chains also undergoes similar two stages to form stable mesoglobules during the heating. In contrast, the triblock chains associate at similar to 30 degrees C to form polymeric micelles.